HT9200A problem (DTMF)
Dennis Plunkett email (remove spam text)
At 21:11 28/10/99 -0400, you wrote:
>> > You cannot filter the DC jump out of the signal, because the jump
>> has wide
>> > spectrum including DTMF frequencies band.
>Choosing the smallest value of coupling capacitor that will still pass
>the tones can help a lot. This forms a high-pass filter so the "thumps"
>become more like "clicks". Most devices that receive DTMF will not be
>affected by such clicks. Clamping the other side of the capacitor to the
>ground (or whatever DC bias is normally present) before stopping the tone
>would remove the disturbance completely.
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Lets take one step back,
Where do you hear these noises?
Is it on the line?
Is it at the handset?
As long as the levels of the "Pops and clicks" is not higher than the
maximum line level, then there is NO PROBLEM.
At the handset of a phone is different!
Here you have to think like a phone! Which by the way at this point is
somewhat different than you may contemplate. (What the !W#$% is he talking
When you dial, what do you here?
1/ Dial tone
2/ First digit goes out and you here this and hear it break dial tone corre
BUZZT go straigt to jail do not pass go do not collect $
When you dial you do not here what goes to line or what is even on the line.
The phone taps off the DTMF at a LOWER (Spelt softer) level than that, that
goes to line (Sometimes up to -4dBm and -7dBm is typical (This is too high,
should be -10dBm), and MUTES the receive audio from the exchange. Then
after the digit is complete, you get connected to the line again, all pops
and clicks removed!
So now you ask about stored digits.
Again the same applies, except that you are NOT connected back to the line
In such a case the output from the DTMF chip should be constent between
Now I have some more questions
How do you have this connected? i.e. Is it one its own? Is the DTMF chip
driving a nominal load or buffer amplifier?
We need some more information. I suspect that the implementation may be
incorrect, thus changing the chip to some other device would be like the
Kirham unit (It serves no purpose accoring to Veger)
See also: www.piclist.com/techref/pots/dtmf.htm?key=dtmf
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